I have a Kenmore Coldspot side by side refrigerator that is not hooked up to a water source. The drip pan underneath does fill up with water occasionally, but has done so more lately.
Your fridge may have a problem that causes more water than usual to condense and flow into the drip pan. The best plan is to have a refrigeration specialist check the fridge and recommend a solution. Sooner is better than later as the problem can increase with time and be more expensive to fix.
Do you have a frost free freezer??? Perhaps the water is from the unit defrosting the freezer itself automatically. That is the way mine does but the water that collects in the pan evaporates so I haven't ever had to empty it.